The arrows indicate the line of action of the muscles that control the shoulder. The supraspinatus moves the humerus anteriorly. The acromiodeltoid, spinodeltoid, and teres major move the humerus posteriorly. Since the teres major inserts on the medial shaft of the humerus it can also rotate it. Can you locate the above muscles on the cat on the left? Click on the numbers to test yourself. Compare the orientation of the humerus in the two pictures. Which muscle(s) would have to contract to move it to the orientation shown here? Back